It is known to remove the nitrogen oxides from exhaust gases by means of catalysts suitable for this purpose together with a supply of ammonia. The exhaust gas containing nitrogen oxide is converted into nitrogen in the process so that the NO originally present is converted into an inert substance.
EP 0 174 907 A2 discloses a process in which both nitrogen oxide and sulphur dioxide can be removed from exhaust gases. In this process, a solution of nitric acid is added to the exhaust gas in a so called denitrating zone. This procedure adjusts the NO/NO.sub.2 ratio to such a value that the dinitrogen trioxide formed can easily be concentrated as nitrosyl sulphuric acid in several successive sulphuric acid absorption stages. From the nitrosyl sulfuric acid a dilute nitric acid and a nitrous-free sulphuric acid are prepared in a separate step of the process at an elevated temperature and with the addition of air, and this nitric acid and sulphuric acid are used again in the denitrating zone and the sulphuric acid absorption stages. This process requires about 7 to 8 absorption towers for concentrating the dinitrogen trioxide as nitrosyl sulphuric acid and therefore involves considerable technical expenditure.
DD 212 495 A1 describes a process for the removal of nitrogen oxides from exhaust gases by successive washing with water and with dilute nitric acid combined with absorption in concentrated sulphuric acid. This process in in principle similar to the one described above but is carried out by a modified procedure.
Although substantial removal of nitrogen oxides from exhaust gases can be achieved by the processes described above, it is not possible to evaluate the nitrogen oxide content of the exhaust gas directly without intermediate stages of working up for the recovery of trivalent, tetravalent or pentavalent nitrogen oxides.
It is therefore an object of the present invention to provide a process which does not have the disadvantages of the processes described above.